I am no genetics expert, but it has been my experience that black crossed with lilac, chocolate and blue produced a more even, more consistent color, than dilute to dilute.
Many years ago, when there weren't so many blues, and chocolate Mini Rex were even more rare, it was quite common to cross black with both lines to "firm up" the color, because too many dilutes, such as lilac, were being produced, and at the time, those couldn't be shown.
I don't in my memory, ever remember a lilac taking best of breed. That's not to say it can't be done, and the quality has vastly improved since I have been away.